1. Field of the Invention
The invention relates to a ballast for low-pressure discharge lamps, comprising a controllable inverter circuit for the generation of a high-frequency supply voltage for a discharge lamp, a lamp inductance connected to the inverter circuit, a lamp parallel capacitor which is serially connected to the lamp inductance and connected in parallel to the discharge lamp, and a preheating circuit supplying heating current to the electrodes of the discharge lamp.
2. The Prior Art
Such a ballast is known, for example, from what has been disclosed in DE 199 20 030 A1. Using the preheating circuit the electrodes of the discharge lamp designed in the form of heating coils are preheated so that a thermionic emission is initiated. For the ignition of the discharge lamp the frequency of the supply voltage is varied from a preheating frequency to an operating frequency with the help of the controllable inverter circuit. This will cause resonance in the series resonance circuit formed by the lamp inductance and the lamp parallel capacitor so that an ignition voltage is applied to the discharge lamp which is sufficient for ignition purposes.
The prior-art ballast is equipped with a heating transformer the primary winding of which is connected in series,with the lamp parallel capacitor. The secondary windings of the heating transformer supply heating current to the electrodes of the discharge lamp said electrodes being designed in the form of heating coils. The preheating voltage applied to the primary winding of the heating transformer is thus exclusively governed by the voltage drop across the lamp parallel capacitor. During the transient period when the preheating frequency changes until the operating frequency is reached this voltage increases causing the heating current to go up as well. However, the heating current is limited as a result of the saturation occurring in the heating transformer. Upon ignition of the discharge lamp the voltage impressed on the lamp parallel capacitor collapses and drops to the operating voltage level of the lamp. The heating current flowing through the heating coils of the electrodes when the lamp's operating state is reached decreases accordingly.
A drawback with such prior-art ballast is, however, that even during the ongoing operation of the discharge lamp a heating current is constantly applied which leads to higher power consumption. This is due to the fact that during lamp operation the primary winding of the heating transformer is constantly supplied with reactive current flowing through the lamp parallel capacitor. Another disadvantage to be associated with the prior-art ballast is experienced when the discharge lamp is switched on causing an undesirably high heating current to flow through the cold heating coils of the discharge lamp's electrodes. The heating current is only be limited by the saturation of the heating transformer. In the event the prior-art ballast is used with discharge lamps having particularly sensitive electrodes damage of the heating coils may occur.